As exemplified by whatwg/html#6064, #9746 and somewhat #2463, it might be a good idea to move Feature Queries out of css-conditional into a separate module maintained by the CSS WG but without ”CSS“ in its title, just like Media Queries.

The @supports at-rule would remain in css-conditional, but what it accepts would be found in a new spec. Most of the content of current drafts for Level 4 (and some of Level 5) is in fact extensions to the support queries, so would need to go into the new document.

That new specification could then, at least in principle, also introduce queries for feature support in other Web technologies like WOFF, ECMAScript, HTML, HTTP, WASM, SVG, PNG, JPEG, MP4 etc.pp.

PS: #10433 is slightly related.